Development of explosive devices, and development of protective measures against their effect, involves an understanding of the behaviour of the device as it explodes. Most explosive weapons generate fragments, which are one of the principal means by which the weapon causes damage. It is therefore desirable to understand the nature and behaviour of the fragments generated by an explosive device as it explodes. For example, understanding the behaviour of fragments close to the explosion helps the analyst to understand the likelihood of the explosion causing collateral damage, i.e. damage other than damage to its intended target. Understanding that likelihood and the behaviour of the fragments helps the analyst to change the design of the explosive device in order to reduce the risk of collateral damage. More generally, understanding the behaviour of the fragments helps the analyst to change the design of the explosive device in order to better control that behaviour.
The principal method presently employed to characterise the fragmentation uses strawboards. Strawboards are large blocks of a material of known shape, density and mass. Strawboards have been used since at least the 1950s to understand energy levels in fragmentation. In a typical assessment, strawboards are arranged around the explosive device. The device is detonated, sending fragments at high velocities into the strawboards. The fragments penetrate the strawboard material. The depth of penetration depends principally upon the kinetic energy of the fragment; knowledge of the depth of penetration enables the energy lost from the fragment in reaching that position to be calculated, and hence the original kinetic energy of the fragment to be deduced. After the explosion, an investigator carefully breaks up the strawboard material and records the position (including the depth) of all fragments in the material. The recorded positions, together with knowledge of the position and orientation of the strawboards themselves, is used to infer the distribution of fragments arising from the explosion Information regarding the mass and form of each fragment can be inferred, as well as knowledge of the direction in which it was ejected from the explosive device. That enables a map of the distribution of mass and energy from the explosion to be calculated.
However, use of strawboards can be problematic. For example, the impact of a fragment on the strawboard can cause the fragment itself to break into smaller fragments. “Strike Line” analysis of the smaller fragments is then required: fragments with final resting positions in the strawboard that are approximately along a line are assumed to have come from a single original fragment. Also, the strawboard material itself is dense and if knocked can be easily damaged. Material can be lost, or not retained, during—or after—the explosion. The fragmentation analysis requires the strawboard packs to be dismantled and the fragments carefully picked out, which is a labour-intensive process, costly in time and resources. There are also Health and Safety issues associated with manual handling of strawboards/packs.
Witness plates are a known alternative to using strawboards for analysis of fragmentation of an explosive device. A witness plate is a sheet of metal. Like the strawboard, witness plates are positioned around the explosive device, at known locations and orientations, and the device exploded. Fragments from the explosion penetrate or perforate the plate. The thickness and material properties of the plate are known, and so the energy required to penetrate or defeat (i.e. perforate) it can be calculated. That required energy provides a minimum, threshold, kinetic energy for the fragment doing the penetration. From the position and orientation of the witness plate, and of the penetration hole on the witness plate, the direction of ejection of the fragment penetrating the plate can be calculated. Thus, a survey of the position of all of the holes in all of the witness plates can be used to calculate a minimum threshold map of the energy distribution of the fragments generated by the explosion. Repeating the experiment using witness plates of different thicknesses, and hence different threshold energies for penetration, enables a full distribution of mass and energy from the explosion to be calculated.
However, it will be appreciated that, although noting the locations of penetration holes is less onerous than dismantling a strawboard, it is still labour-intensive, and the need for repeated experiments results in prior-art use of witness plates being time consuming and costly. Manual measurement and transcription of data requires handling and manipulation of plates with potentially sharp edges. Accuracy can be limited where plate damage has occurred that has resulted in curvature or other distortion.
It is also known to measure the strike position of fragments ejected by a test warhead specifically designed to eject fragments for measurement and assessment purposes. Such pre-formed, “fixed mass”, fragmenting systems can be used to generate highly accurate positional data which, when combined with the relevant associated velocity characteristics, offers precise characterisation of energy across the fragment beam. Precision controlled warheads can produce such an output, which results in uniform damage area per strike; however, naturally fragmenting systems break into fragments that are irregular in mass, form and in the area over which they cause damage. It would be advantageous to be able to characterise irregular fragmentation.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved method and apparatus for characterising fragmentation of an explosive device.